Pair hidden triple
Three pairs of one symbol confined to three cells lock those cells
How to use it
Three pairs of one symbol — say A1, A4 and A6 — whose homes all lie inside the same three cells. The three pairs are then distributed over those three cells and nothing else: every other pair is blocked there, and any candidate letter or digit that cannot participate in one of the three pairs dies at those cells.
Build the homes table for one letter (or digit) at a time: list each unplaced pair with its candidate cells. Three pairs whose homes union to three cells form the lock. It fires late in a solve, when a symbol is one or two placements from pinned.
Why it is sound
Three unplaced pairs of one symbol whose homes all lie inside the same three cells must be distributed over those cells in every solution (each pair needs a home and each cell holds one pair, so homes inside three cells force the three pairs into them). Blocking every other pair there is sound.
A worked example
Digit 6's pairs A6, F6 and G6 have all their remaining homes inside three cells: r4c1, r5c4 and r6c6 — three cells that between them must absorb the three pairs, one pair each. The leftover E6 has nowhere left to go: it dies at r6c6.